Marginal hypothyroidism, in which thyroid hormone concentrations are normal, but basal TSH or TRH stimulated TSH concentrations are elevated, affects at least 5% of the adult female population. While overt hypothyroidism is known to be associated with psychiatric and neurological illness, little is known about the neuropsychiatric consequences of marginal hypothyroidism. Preliminary findings indicate that it may be a potentially modifiable risk factor for both affective illness and cognitive dysfunction. Our main objective in this study will be to characterize the nature and degree of cognitive and affective dysfunction in non-psychiatric subjects with untreated marginal hypothyroidism. Healthy female subjects recruited from a variety of sources will have their thyroid function screened with basal TSH and thyroid hormone measurements. Sixty subjects identified in this manner to have grade 2 hypothyroidism (elevated basal TSH, normal Free Thyroxin Index) will have their thyroid status confirmed with TRH infusion testing, and in addition will receive comprehensive neuropsychological and psychodiagnostic assessments. A matched group of euthyroid subjects from the same subject pool will undergo identical testing. Neuropsychological test scores and the lifetime prevalence of psychiatric illness of confirmed marginally hypothyroid and euthyroid subjects will be compared statistically.